I’m not sure that R+D needs to be at the operator versus manufacturer level. For instance, manufacturers not operators bear the R+D cost of better diesel engines.
You can retain (relatively) small fleets if the manufacturer is amortizing the cost of technology across vehicles sold.
It really depends on whether the AV companies decide to focus purely on manufacturing and developing AVs as a product (profit at the point of sale), offer them as a service (similar to "power by the hour" in aerospace), or decide to become robotaxi/truck fleet operators to capture more profit.
You can retain (relatively) small fleets if the manufacturer is amortizing the cost of technology across vehicles sold.